It's a bit confusing... I've never seen blender's armature mirror work like I think they wanted it to.
(workaround) Select all the bones you want to mirror in edit mode, hold shift+d to duplicate them, then right click to avoid movement. Press g, then x, to move the bones only along the x axis. Scale the bones on the x axis (press s, then x ). Hold shift while doing so to snap the scaling to even values, and stop at x=-1. Finally, zoom in on the center where the bones meet with their parents and hit g, then x, to position them just right.